In recent years, electronic devices (compound semiconductor devices) in which GaN layers and AlGaN layers are sequentially arranged over substrates and in which the GaN layers are used as electron travel layers have been under active development. One of such compound semiconductor devices is a GaN high-electron mobility transistor (HEMT). The use of the GaN HEMT as a switch for inverters for power supplies enables both the reduction of on-resistance and the enhancement of dielectric strength. Furthermore, the GaN HEMT may reduce standby power consumption and may increase operating frequencies as compared with Si transistors. These enable the reduction of switching loss and the reduction in power consumption of inverters. GaN HEMTs enable downsizing as compared with Si transistors equivalent in performance to the GaN HEMTs.
In a GaN HEMT including a GaN layer used as an electron travel layer and an AlGaN layer used as an electron supply layer, strain due to the difference in lattice constant between AlGaN and GaN is caused in the AlGaN layer. Therefore, piezoelectric polarization occurs and a high-concentration two-dimensional electron gas (2DEG) is obtained. Accordingly, the GaN HEMT is suitable for high-power device applications.
However, it is very difficult to produce a GaN substrate with good crystallinity. Therefore, for example, conventional GaN compound semiconductor layers such as GaN layers and AlGaN layers have been formed mainly over a Si substrate, a sapphire substrate, or a SiC substrate by heteroepitaxial growth. In particular, large-size, high-quality Si substrates are readily available at low cost. Therefore, structures formed by growing GaN layers and AlGaN layers over a Si substrate are under active investigation.
However, there are large differences in thermal expansion coefficient between a GaN layer, an AlGaN layer, and a Si substrate. On the other hand, high-temperature treatment is used to epitaxially grow the GaN layer and the AlGaN layer. Therefore, the Si substrate becomes warped or cracked due to a difference in thermal expansion coefficient during such high-temperature treatment in some cases. In order to solve problems caused by such a difference in thermal expansion coefficient, the following technique is being studied: a technique for providing buffer layers, having a super-lattice structure in which two-types of compound semiconductor layers different in composition from each other are alternately stacked, between a GaN layer, an AlGaN layer, and a Si substrate.
However, in conventional compound semiconductor devices including buffer layers having a super-lattice structure, it is difficult to sufficiently suppress cracking, warpage, and the like. Furthermore, it is difficult to allow an electron travel layer and electron supply layer formed on such a super-lattice structure to have good crystallinity.
The following technique is also being studied: a technique for forming a layer having a thermal expansion coefficient close to that of a GaN compound semiconductor layer on the back surface of an Si substrate. However, for this conventional technique, warpage or the like occurs when this layer is formed. Since the occurrence of such warpage causes variations in substrate temperature during the formation of an electron travel layer and an electron supply layer, it is difficult to obtain desired properties. Japanese Laid-Open Patent Publication Nos. 2010-228967 and 2011-119715 are examples of related art.